The present invention relates generally to industrial turbine engines, and more specifically, to combustor therein.
Industrial power generation gas turbine engines include a compressor for compressing air that is mixed with fuel and ignited in a combustor for generating combustion gases. The combustion gases flow to a turbine that extracts energy for driving a shaft to power the compressor and produces output power for powering an electrical generator, for example. The turbine is typically operated for extended periods of time at a relatively high base load for powering the generator to produce electrical power to a utility grid, for example.
Over the past ten years there has been a dramatic increase in the regulatory requirements for low emissions from turbine power plants. Environmental agencies throughout the world are now requiring low rates of emissions of NOx, CO and other pollutants from both new and existing turbines.
Traditional turbine combustor use non-premixed diffusion flames where fuel and air freely enter the combustion chamber separately and mixing of the fuel and air occurs simultaneously with combustion. Typical diffusion flames are dominated by regions that burn at or near stoichiometric conditions. The resulting flame temperatures can exceed 3000.degree. F. (1650.degree. C.). Because diatomic nitrogen reacts rapidly with oxygen at temperatures exceeding about 2850.degree. F. (1565.degree. C.), diffusion flames typically produce relatively high levels of NOx emissions.
One method commonly used to reduce peak temperatures, and thereby reduce NOx emissions, is to inject water or steam into the combustor. Water or steam injection, however, is a relatively expensive technique and can cause the undesirable side effect of quenching carbon monoxide (CO) burnout reactions. Additionally, water or steam injection methods are limited in their ability to reach the extremely low levels of pollutants now required in many localities. Furthermore, this approach cannot be used in installations where water or steam is not available, for example, remote pipeline stations
Due to these limitations of traditional diffusion flame combustor, lean premixed gas turbine combustor were developed. Lean premixed combustors can achieve very low NOx and CO emissions without diluent injection. Lean premixed combustors mix the fuel and the air prior to combustion thus eliminating the high temperature conditions which lead to NOx formation. This reduction in emissions, however, is achieved at the expense of simplicity and cost. Premix combustors can cost five to ten times more than traditional diffusion flame combustors, as premix combustors frequently include multiple fuel injectors or fuel nozzles, as well as multiple fuel manifolds, multiple purge manifolds, and multiple fuel control valves. Furthermore, premix combustors typically have multiple modes of operation. Lean premixed combustors can operate in a premixed mode and achieve the low emissions of premix combustion only over a narrow load range, typically near base load. At reduced loads, however, premix combustors must often be operated as diffusion flame combustors, due to flammability limits. This need for mode switching adds cost and complexity to the combustion system.
Therefore, it is apparent from the above that their exists a need in the art for an improved gas turbine combustor that combines the low-cost and simplicity of operation of a diffusion flame combustor and the reduced emissions of a premixed combustor.